This invention generally concerns synchronous rectifiers and more particularly relates to a means for providing a simple controller for synchronous rectifiers during start-up into pre-biased output voltage.
Whenever a converter employs synchronous rectification a problem arises during the start-up (turn-on) period if the output voltage of the converter is non-zero, but has some positive voltage. This is a common problem in systems where different voltages are required and the sequence in which they are enabled is well defined. Typically, the converter with the lowest voltage is started first and the converter with the highest voltage is started last. Parasitic diodes in integrated circuits (ICs) cause the converter with the higher voltage (for example, 3.3 V) to xe2x80x9cseexe2x80x9d a lower voltage on its output from the lower output voltage converter (for example, 2.5 V or less), even before it is enabled. If proper control of synchronous rectifiers is not employed, sag in the output voltage of the lower output converter will occur and likely result in a current limit and latch of both converters. A similar problem occurs when converters with synchronous rectifiers are connected in parallel without OR-ing diodes and are started in sequence rather simultaneously.
If possible it would be advantageous to disable the synchronous rectifiers during start-up and then enable them again. Unfortunately, the solution to this problem is not that simple. Even if synchronous rectifiers were disabled during start-up, sudden enabling of them after the output voltage reached its nominal value causes an undesired huge negative current of the converter and a drop in output voltage, particularly if no load or a minimum load is applied.
Turn-off transients are also important system concerns. If the synchronous rectifier, connected across the output, commonly through an inductor, is not disabled or well controlled during this transition, a negative voltage at the output can occur due to resonance between an inductor and an output capacitor in a loop with the synchronous rectifier. Since it is a current bi-directional device, the synchronous rectifier allows negative inductor current flow that results in a negative output voltage, which, in most cases will destroy the load. This problem may also occur when two or more converters are connected in parallel.
Accordingly, what is needed in the art is a system and method that provides improved control of synchronous rectifiers during these transient conditions associated with a power converter.
To address the above-discussed deficiencies if the prior art, the present invention provides for use in a power converter having a power switch and a synchronous rectifier device coupled between input and output thereof, a control circuit, method of disabling a synchronous rectifier device and power converter employing the control circuit and method.
In one embodiment, the control circuit includes a synchronous rectifier controller, coupled to said synchronous rectifier device, that senses a time derivative of the output voltage of the power converter and disables said synchronous rectifier device when said time derivative is negative and greater than a predetermined magnitude.
In one embodiment of the present invention, the synchronous rectifier device is coupled across the output of the power converter via an inductor. However, the present invention is equally applicable to a synchronous rectifier device located at any position associated with the power converter.
In one embodiment of the present invention, the power converter further includes a plurality of synchronous rectifier devices. The synchronous rectifier controller is adapted to disable at least one of the plurality of synchronous rectifier devices. In a related, but alternative embodiment, the power converter further includes a plurality of power switches. The present invention is equally applicable to any power topology either non-isolated or isolated employing at least one synchronous rectifier device.
In one embodiment of the present invention, the synchronous rectifier controller comprises at least one logic gate to enable or disable corresponding at least one synchronous rectifier; and differentiating means for sensing said time derivative of said output voltage. The differentiating means comprises a comparator having an inverting input and a non-inverting input, a resistor network wherein the resistance of each said resistor is chosen to determine the steady state voltages at said inverting and non-inverting inputs of said comparator and said predetermined magnitude of transient and a capacitor coupled between said output of said power converter and said one input of said comparator, the capacitance of said capacitor together with resistance of said resistors connected to said one input of said comparator define a time constant, said time constant is chosen of sufficient length to allow for proper operation of said power converter during turn-on into said output with a nonzero voltage present. Of course, other controllers capable of sensing the negative derivative of the output voltage and disabling the synchronous rectifier device under certain conditions are well within the broad scope of the present invention.
In one embodiment of the present invention the synchronous rectifier controller further comprises an ON/OFF circuit for disabling the comparator after converter is turned-on and output voltage is in regulation. The ON/OFF circuit disables the comparator after predetermined time from the moment when said converter is turned-on. In related, but alternative embodiment, the ON/OFF circuit senses the output voltage of the power converter and disables the comparator when the output voltage reaches predetermined percentage of its nominal value during turn-on sequence.
Those skilled in the art should appreciate that they can readily use the disclosed conception and specific embodiment as a basis for designing or modifying other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form.